Conventional camera optics are derived from the eyes of vertebrates, wherein a single lens system captures light through a large aperture and focuses it onto a concave retina. Single aperture optics have the advantage of good spatial resolution and efficient light capture, but they are disadvantaged by their relatively large size and limited field of view. Vertebrates evolved to overcome the field of view limitation by gimbaling the eye in its socket, and by restricting the high resolution capability to the fovea centralis. Camera developers have attempted to extend the high resolution capability beyond the fovea by introducing additional optical elements to reduce the distortion that results from focusing an inherently concave image onto a flat film surface. The classical solution of adding spherically shaped optical elements does not eliminate distortion, and it yields a lens that is heavy and long. An example of a classical high resolution, wide angle lens is described by Momiyama in U.S. Pat. No. 4,437,735. The lens extends 20 inches beyond the image plane and uses 13 powered optical elements of different sizes, shapes, and materials to correct for aberrations. A modern approach, described by Gohman and Peterson in U.S. Pat. Nos. 7,545,586 and 7,768,715, is to introduce aspheric elements and an intermediate image, which unfortunately retains a slight concavity. Though this design form improves performance while reducing distortion to an acceptable level at the final image plane, it can still require more than a dozen powered optical elements.
The excessive use of aspheric surfaces in modern wide angle systems highlights a fundamental contradiction in conventional optical design principles: Aberrations increase exponentially as the field angle and pupil size increase laterally away from the central optical axis; designers can only correct for these laterally induced aberrations indirectly by adding and altering optical surfaces sequentially along the central optical axis. It will be shown that this design contradiction can be overcome by adding optical surfaces laterally away from the central optical axis. This enables the designer to optimize in discrete increments along multiple axes to correct for the laterally induced aberrations directly.
Another disadvantage of the high resolution, single aperture lens is its need for focus adjustment to image objects at different distances. The problem is especially acute at close range and has prompted inventors to adopt various schemes to automate the focus adjustment process. One example of an auto focus system is described by Watanabe et al. in U.S. Pat. No. 7,184,090. It engages in a focusing operation while sending out an image capturing signal. It then settles on the focus position that achieves the highest value in image contrast. As with all such auto focusing schemes, it cannot overcome the inherent design limitation of a single aperture lens: Objects at different depths of field cannot be brought into focus simultaneously.
Wide angle single aperture lens systems are also unable to generate images with uniform intensity. The image intensity generally degrades with field angle as the cosine to the fourth power when viewing a flat target. Thus, the image intensity at 60 degrees off-axis is less than 7% of the image intensity at 0 degrees. Optical designers routinely attempt to overcome this inherent limitation by oversizing and distorting the entrance pupil for the off-axis fields while positioning a second pupil plane far back in the system to serve as the aperture stop and iris. A double pupil plane design can limit the intensity degradation to about 50% but cannot eliminate the problem entirely.
Single aperture lens systems are inherently prone to stray light degradation, even when designed with an intermediate field stop. The stray light is generated by intense, out-of-view radiation striking the first optical surface and scattering toward the final image plane. Since the front surface of the first lens is visible from every point on the image plane, the scattered radiation cannot be blocked. Space craft mounted star trackers have attempted to deal with this problem by using very long baffles to limit the sun's illumination angle. However, even with the extended baffle, the best systems are restricted to tracking stars that are more than 30 degrees off-axis from the solar line of sight. Stars that fall within the 30 degree sector are not bright enough to overcome the solar stray radiation and cannot be tracked.
There is the need in many autonomous surveillance and robotic navigation applications for a high resolution, wide angle imaging system that remains in focus through all depths of field, that is impervious to stray light, and that generates a distortion free image with uniform intensity. Such a system can be derived from the most popular eyes found in nature, the multiple aperture compound eyes of arthropods (i.e. insects and crustaceans). Compound eyes are formed from a convex array of micro-lenses (or lenslets) that collectively capture light through a very large field angle. The inherent advantage is that each sector of the field is separated into tiny zones that are imaged independently through lenslets positioned in the direction of the incoming image light. Since the aperture diameter and field angle of each lenslet are small, the corresponding optical aberrations are small. Since each aperture is oriented along its own optical axis pointing toward its own target sector, the image intensity remains uniform from aperture to aperture across the entire field of view. The composite image generated by the array of lenslets is distortion free and remains in focus at all depths of field because each lenslet captures a very small section of the optical wavefront emanating from the object. The smaller the wavefront sections, the flatter they become until all objects appear to be at infinity. This is why arthropods have no need for a focusing mechanism.
Natural compound eyes can be divided into two general categories: apposition and superposition. In the apposition eye a simple corneal lenslet focuses light directly onto a nearby receptive rod called a rhabdom. The two components constitute an ommatidium, of which there are thousands. Only a small cone of light along the axis of each ommatidium is detected. Light entering from outside the cone angle is absorbed in surrounding pigment cells. The spherical layout of the array enables adjoining lenslets to view adjacent fields. Though each lenslet image is inverted, in mosaic form the composite image appears erect because the lenslet viewing sectors are so small.
The architecture of the superposition eye varies slightly from that of the apposition eye. The superposition eye includes a meniscus shaped shell of long crystalline cones, a clear zone, and a convex rhabdom layer separated from the cones by a distance equal to half the radius of curvature of the outer meniscus surface. The cornea of each crystalline cone focuses incoming light within the cone and then collimates it in the latter part of the cone. The cone therefore acts as both a Keplerian telescope objective and an afocal eyepiece. Since the array of cones form a meniscus structure, the collimated light of a common field angle converge from adjoining cones to a single point on the confocal contour of the rhabdom layer. Thus the light from all of the cones separate according to field angle and then superpose on the rhabdom surface to produce a single, upright image. The superposition eye is a true pupil imager because the beam convergence points on the rhabdom surface are actually tiny exit pupils. Since light from a large number of cones contribute to each of these convergence points, the effective sensitivity of a superposition eye is increased significantly relative to an apposition eye. This is why apposition eyes are found primarily on diurnal arthropods, such as butterflies, and superposition eyes are found primarily on nocturnal arthropods, such as moths.
Despite the inherent light capturing advantages of the superposition architecture, artificial compound eyes are more commonly derived from the much simpler apposition format. Duparré et al. describe a flat lenslet array artificial compound eye the size and shape of a credit card (see Duparré et al., Applied Optics, August 2004, pp 4303-4310, vol. 43, No. 22). Hirasawa et al. describe another flat array variation in U.S. Pat. No. 7,718,940, as do Tamaki et al. in U.S. Pat. No. 7,865,076. The flat design attribute is beneficial in that it enables the use of flat lenslet arrays, which are routinely manufactured in a variety of ways (see for example Fadel et al., U.S. Pat. No. 6,967,779). The flat design also matches well to flat mosaic detector arrays, which are easy to manufacture and readily available. However, the flat design attribute limits the field of view and causes the image intensity to degrade significantly with field angle. Toyoda et al., in U.S. Pat. No. 7,974,015, describe a method for overcoming these limitations using prisms to redirect light into the flat lens array from oblique angles. Another variation of a flat lenslet array system is described by Gurevich et al. in U.S. Pat. No. 7,187,502. This system uses a second flat lenslet array of a different pitch to increase the magnification of the image. In all these designs, some method of signal processing is required to rearrange the sub-images into a more coherent full image.
Lee and Szema describe an artificial apposition array compound eye that closely mimics the design found in nature (see Lee and Szema, Science, November 2005, pp 1148-1150, vol. 310, No. 5751). The lenslet array is convex in shape, and the right from each lenslet is focused onto a convex surface. Unfortunately, the design requires a convex shaped detector array of extremely small size to capture the image.
Another curved apposition compound eye concept is described by Sweatt and Gill in U.S. Pat. No. 7,286,295. In this concept the lenslets have power on two surfaces and are preferred to be aspheric to correct for optical aberrations. The lenslets are made from polymethyl methacrylate and are optimized for a single wavelength. The lenslets are separated laterally along the array by a spacer baffle that prevents cross-talk between cells. The lenslets focus a series of inverted sub-images onto a dome shaped, coherent fiber optic bundle that is supposed to transport the sub-images onto a flat detector array. However, the fiber optic bundle is not tapered, and so the fiber tips are beveled along the peripheral regions of the dome. The bevels prevent light capture along the axes of the peripheral lenslets and encourage stray light capture from oblique angles. Gaps and overlaps in the sub-images are controlled by the spacing of the lenslets and the curvature of the array. Since the sub-images are each inverted, the composite image must be constructed digitally by post-processing.
A curved apposition compound eye free of fiber optics is described by Jiang and Dong in U.S. Pat. No. 7,672,058. They incorporate microfluidic devices into the array to tune the focal lengths of each lenslet in response to a variety of environmental factors. The adjustable focal length appears to compensate for the variation in distance to the flat focal plane surface.
An artificial superposition array compound eye is described in U.S. Pat. No. 7,376,314. In this concept two lenslet arrays are hot press molded into a convex, meniscus form. The lenslets are paired to operate as afocal Keplerian telescopes that focus, collimate, and bend the incoming light. The mensicus form enables the collimated light from adjacent lenslets to be directed toward a common point on the convex surface of a fiber optic imaging taper. In this manner all of the lenslets work together to form a single, upright, high intensity image on top of the taper. The taper transfers the upright image to a flat detector array; no digital post-processing is required. The fiber tips of the taper are each cut perpendicular to the fiber axes, so only image light from the correct angles are captured by the fibers. A honeycomb louver baffle is positioned between the lenslets and the taper dome to block ghost images. The diameter of the honeycomb cells set the effective pupil size of the optics. A typical cell diameter encompasses 100 lenslets out of the 30,000 lenslets in each array, thereby increasing its sensitivity by a factor of 100 over an equivalent apposition eye.
A hybrid superposition-apposition artificial compound eye is described in U.S. Pat. No. 7,587,109. Since hybrid compound eyes are not found in nature, the design architecture had to be invented from first principles. The hybrid combines the sensitivity of the superposition eye with the resolution of the apposition eye, and it generates a single, upright image without the need for post-processing. The invention makes use of a honeycomb field stop baffle to block stray light and ghost images, and it uses a fiber optic taper dome to transfer the convex image onto a flat detector array.
The main shortcoming in the hybrid superposition-apposition artificial compound eye is its reliance on the fiber optic taper dome to generate a flat image. Fiber optics increase the system cost, limit the minimum detector pixel pitch, and prevent the optics from being mounted directly to camera bodies designed for conventional lenses. With regard to cost, it is well known that assembling millions of fibers into a taper is labor intensive, and bonding the taper to a detector array is a highly skilled art. The taper must be bonded carefully to an active detector within a fixture that enables the technician to view the image while rotating the taper. The technician must rotate the taper to eliminate all moire interference patterns and then cure the bonding material to fix the taper in place.
The moire interference problem limits the ultimate size of the detector pixel. Two fibers must fit across each pixel to eliminate moire, and the fibers must be larger than about 3 microns to preserve light transfer efficiency. This implies that the pixel pitch must be greater than 6 microns. Since optical size and resolution performance scale with pixel pitch, the 6 micron pitch becomes the ultimate limit in size reduction and resolution. The most advanced detector arrays now have pixels approaching 1.1-1.4 microns in pitch, which implies that the fibers must be about 0.5 microns in diameter to eliminate moire. It is unlikely that fibers can be manufactured reliably at this diameter. Certainly waveguide effects in these fibers would limit light transfer through the taper since the uncladded core diameter of the fiber would be smaller than the wavelength of light.
Fused fiber optic tapers are not available in infrared materials, which limits the wavelength range of the optics. Even if infrared transmitting tapers were available, they could not be bonded to uncooled, infrared micro-bolometer arrays because they would damage the bolometer structure. It would also be unwise to bond tapers to expensive, cooled infrared detector arrays due to the risk of damaging the passivation layer beneath the contact surface.
It is therefore concluded that a more useful artificial compound eye is one that does not rely on fiber optics to transform a convex image into a flat image. A new design concept must be invented that preserves the inherent advantages of artificial compound eyes while incorporating a versatility that enables the new optics to mount directly onto conventional camera bodies.